There is now strong evidence to indicate that the gaseous anesthetic, nitrous oxide, causes an increased incidence of tumors in operating room and dental personnel. This evidence is based mainly on a recent retrospective survey which showed that female dental chairside assistants working with nitrous oxide had twice the tumor incidence of similar groups not working with that anesthetic. Previous surveys supported this finding. Although nitrous oxide is the most widely used inhalational anesthetic throughout the world and has numerous other commericial uses, only two limited studies of its carcinogenic potential have so far been completed. The methods used in these negative studies, however, are generally considered too insensitive to give conclusive results. Thus, in light of the new epidemiological evidence, it seems imperative that the carcinogenic potential of nitrous oxide be studied as soon as possible in a complete bioassay. Therefore, the primary aim of this project is to perform in vivo studies to determine whether nitrous oxide is carcinogenic to experimental animals. A secondary minor aim is to use the results obtained to predict whether there are feasible methods for reducing the full histopathological examination in carcinogen bioassays. Preliminary studies will be performed to determine the maximum tolerated dose (MTD) of nitrous oxide. Groups of 150 Swiss/ICR mice will be exposed for 18 months to either the MTD of nitrous oxide or subfraction of MTD. A similar control group will be exposed to air under the same experimental conditions. At or after this period, animals will be sacrificed and complete necropsy performed. Thirtytwo tissues will subjected to full histopathological examination. The number and types of tumors found will be documented and statistical comparisons made among groups. These investigations should determine whether nitrous oxide is carcinogenic to Swiss/ICAR mice. It should then be possible to infer similar conclusions about risk to operating room personnel and to improve experimental design for future carcinogenicity studies.